The term "vitamin D dependent disorders" refers to disorders which can be treated or prevented by the administration of compounds having vitamin D activity, such as vitamin D.sub.3 or derivatives, in particular hydroxylated derivatives thereof, e.g. calcitriol or calcipotriol. Examples of such disorders are hyperproliferative skin diseases such as psoriasis, basal cell carcinomas, disorders of keratinization and keratosis; neoplastic diseases such as leukemia; disorders of the sebaceous glands such as acne and seborrhoic dermatitis; osteoporosis; hyperparathyroidism accompanying renal failure; and diseases which require modulation of the immune system, such as transplant rejection and graft vs. host disease.
However, vitamin D activity is often coupled with an undesirable level of toxicity. It is important to obtain compounds that have vitamin D activity sufficient to combat vitamin D dependent disorders, which have toxicity as low as possible compatible with activity.